How do ultra-precision balances measure the mass of microscopic materials using the piezoelectric effect?

Ultraprecision balances use the piezoelectric effect to measure the mass of microscopic materials. A crystal oscillator is vibrated by applying a voltage, and its frequency changes as the material sticks to it. By measuring the change in frequency, the mass of the microscopic material and the concentration of a particular gas can be accurately detected.

 

Scales typically weigh mass using the principle of leverage or by measuring changes in electrical resistance. But how do ultra-precise scales weigh microscopic materials such as gas molecules or DNA? To answer this question, we need to understand the piezoelectric effect. Piezoelectricity is the phenomenon in which mechanical strain in a solid material causes an electrical response, which can be categorized into primary piezoelectricity and secondary piezoelectricity. The primary piezoelectric effect is the generation of a voltage when a material is subjected to mechanical strain, while the secondary piezoelectric effect is the generation of mechanical strain when a voltage is applied. Materials that exhibit both of these piezoelectric effects are called piezoelectrics, and crystals are the most common.
The crystal used as a piezoelectric is cut and machined in a specific direction to form a flat disk. Electrodes are then placed on both sides of the disk and a voltage is applied to alternate between the (+) and (-) poles, causing the crystal to vibrate. The frequency of the voltage is matched to the natural frequency of the crystal to make the crystal vibrate in a large range, making it easier to measure the vibration. The natural frequency is the unique vibration frequency of an object, which varies depending on the shape and size of the piezoelectric, even for piezoelectrics of the same material.
As the mass of a crystal oscillator increases, the frequency of the crystal oscillator vibrating at its natural frequency decreases. The frequency of a crystal oscillator is sensitive to very small changes in mass, making it possible to measure the mass of microscopic materials such as gas molecules or DNA. The mass sensitivity of a crystal oscillator is the amount of change in frequency divided by the measured mass, and the mass sensitivity of a crystal oscillator is very large.
The principle of mass measurement with a crystal oscillator can be applied to detect the concentration of a particular gas. When a crystal oscillator is treated with a specific gas, the gas adheres to it and changes its mass, causing the frequency of the crystal oscillator to decrease. At a certain point, the frequency of the crystal oscillator stops decreasing and remains constant. The reason for this constant value is that the specific gas does not adhere to more than a certain amount. The greater the concentration of a particular gas in the mixture, the smaller the frequency will remain constant. The measure of how quickly the gas sticks to the crystal oscillator and the frequency becomes constant is called the reaction time, and the shorter the reaction time, the faster the concentration of the gas can be determined.
However, it is difficult to measure the exact concentration of the target gas you want to measure if a gas other than the one you want to measure is attached to it, and it is not possible to know the concentration of the target gas even if only the target gas is attached. For this reason, it is necessary to measure the frequency change of the crystal oscillator according to the concentration of the target gas in advance. Then, by measuring the frequency change in a mixture of gases without knowing the concentration of the target gas, the concentration of the target gas can be determined. Dividing the frequency change of the crystal oscillator by the concentration gives the sensitivity to concentration.
Measurements with a crystal oscillator can be used not only in the laboratory but also in a variety of industrial applications. For example, in the semiconductor manufacturing process, ultra-precise scales are used to measure the thickness of materials deposited on substrates for quality control. In the field of biosensors, crystal oscillators are used to detect the presence of specific proteins or viruses. This technology plays an important role in medical diagnostics, environmental monitoring, food safety, and many other fields.
Finally, the possibilities for crystal vibrators are endless. Recent research is actively developing new measurement methods and sensors using crystal oscillators. For example, ultra-sensitive nanoscale scales for measuring the mass of nanoparticles and high-performance sensors that operate reliably in extreme environments are being developed. These technological advances will continue to increase the importance of precision measurements using crystal oscillators.